A magnetic bearing, which includes a rotor and a stator concentrically located with respect to each other, typically controls the radial or axial distance between the rotating rotor and the stationary stator. More specifically, adjustable electromagnetic forces generated by current flowing through coils wrapped around the stator, as controlled by a control circuit adjusts distance between the stator and rotor. U.S. Pat. Nos. 4,387,935 and 4,082,376 describe details of the magnetic bearing.
To measure the position of the magnetic bearing rotor and to provide input signals to the control circuit to adjust the forces by adjusting the current through the coils, non-contacting sensors have been used including inductive sensor and capacitance type sensors.
The inductive sensor measures the eddy current generated by sensor coils. U.S. Pat. No. 4,082,376 describes the inductive sensors used in magnetic bearings. In capacitive sensors, a variable capacitor is used to sense gap distance and the resulting capacitance change is converted into an electrical signal indicative of the gap. U.S. Pat. No. 5,864,303 describes a capacitive sensor system for magnetic bearings. A drawback of both types of sensors, however, is that they are prone to electromagnetic interference (EMI) generated by the electro-magnetic coils of the magnetic bearing. Thus they cannot be placed too close to electro-magnetic coils of the magnetic bearing. Further, they tend to drift as operating temperature changes. Therefore, these types of sensors cannot guarantee long term stability due to their sensitivity to temperature and EMI.
Optical position sensors provide position information for magnetic bearings, overcoming the drawbacks mentioned above. In other words, the optical position sensor is insensitive to the EMI generated by the magnetic bearing and can be placed within or very near the electromagnet of the magnetic bearing. Further, the optical position sensor possesses characteristics of long term stability and high resolution.
U.S. Pat. No. 4,456,378 describes an optical sensor for sensing the radial position of a shaft when a portion of a light beam incident on the shaft sensor-target is blocked. The referenced patent proposes the light beam to have a relatively small diameter and be well collimated in order to pass sufficient energy to the detector and in order to provide a high sensitivity to shaft position. When using an optical position sensor, scattering effects must be taken into account as well as calibrating the intensity of the light beam. However, the referenced patent does not recognize design features needed for dealing with the effects of scattered light reflected by the sensor target and the interior of the sensor housing. It also includes a rather complex feedback system requiring shaft motion and sophisticated electronics for calibrating intensity of the light beam.
Thus, it is an object of this invention to provide optical position information for magnetic bearings.
It is a further object of this invention to provide an optical position sensor that possesses long term stability by compensating for degradation of a light beam source or temperature variation.
It is still another object of this invention to provide an optical position sensor that can perform well when scattered light is present either by generation from the light source or by reflection by the shaft sensor-target or the interior of the sensor housing.